Anaerobic substrates

A lot of people have reported not seeing/noticing anaerobicity after 18
months without any form of UG circulation.  I have noticed anaerobicity,
but only noticed it when I gravel vacuum (= rotten egg smell & dark/black
bits in gravel).  I'm just wondering if others do not gravel vacuum &
hence do not notice it, or if it is because they have very heavily planted
& root bound tanks.  My tank that this has occured in is a 25 gal tank
which is not very heavily planted.